Deposition of integrated protective material into zirconium cladding for nuclear reactors by high-velocity thermal application

ABSTRACT

A zirconium alloy nuclear reactor cylindrical cladding has an inner Zr substrate surface, an outer volume of protective material, and an integrated middle volume of zirconium oxide, zirconium and protective material, where the protective material is applied by impaction at a velocity greater than 340 meters/second to provide the integrated middle volume resulting in structural integrity for the cladding.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims priority toU.S. patent application Ser. No. 15/084,577 filed Mar. 30, 2016, whichis a divisional application of and claims priority to U.S. patentapplication Ser. No. 14/548,630 filed Nov. 20, 2014, which is adivisional application of and claims priority to U.S. patent applicationSer. No. 13/670,808, filed Nov. 7, 2012 entitled DEPOSITION OFINTEGRATED PROTECTIVE MATERIAL INTO ZIRCONIUM CLADDING FOR NUCLEARREACTORS BY HIGH-VELOCITY THERMAL APPLICATION.

BACKGROUND 1. Field

This invention relates to integrated protective impregnation depositioninto nuclear cladding.

2. Description of Related Art

The exposure of zirconium cladding to the high-temperature and highpressure water environment in a nuclear reactor can result in thecorrosion (oxidation) of the surface and consequent hydriding (due tothe hydrogen release into the metal from the oxidation reaction withwater) of the bulk cladding, ultimately leading to metal embrittlement.This weakening of the metal can adversely affect the performance,life-time, and safety margin of the nuclear fuel core. Recognizing this,many attempts to coat the zirconium outer surface with one or morelayers of various materials have been tried, for example, Knight et al.,Bryan et al., Van Swam and Lahoda et al. (U.S. Pat. Nos. 6,231,969;5,171,520; 6,005,906; and 7,815,964, respectively). The simple inclusionof an oxidation resistant coating on the zirconium surface can, intheory, protect the zirconium substrate from the reactor environment;however, strong adherence of the coating to the zirconium substrate isproblematic due to a fine oxidation layer that always exists on top ofthe zirconium surface as shown in prior art FIG. 1. These prior artprocesses often result in the coating peeling or spalling off the oxidesurface when the coated cladding is exposed to prior art reactoroperating conditions.

Knight et al. disclose a discrete coating such as Ti₃SiC₂ with less than30% porosity with a thickness between 0.002 inch and 0.005 inch. Bryanet al. disclose initial heating of the cladding from 300° C. to 400° C.,and flame spraying a mixture of zircon, about 30 micrometers, diameterand glass binder, less than 10 micrometers, to provide an intermixeddiscrete coating on the cladding.

Lahoda et al. disclose abrading the surface of the zirconium cladding toremove oxides and surface deposits, and spraying boron, gadolinium,hafnium, erbium, HfB₂, ZrB₂, Gd₂O₃, or Er₂O₃ or their mixtures—allburnable poisons having particle sizes from 1 micrometer to 250micrometers, at a velocity of from 1,500 ft./sec. to 2,500 ft./sec. (457meters/sec. to 762 meters/sec.). This initiates a surface phase changeat the exterior surface of the cladding, so some molecular meltingoccurs (inter-atom bonding or forming craters) and the impactingparticles provide a still discrete impacted surface coating. Van Swanprovides discrete “coatings”/layers of different oxygen contentzirconium claddings, as many as three.

Knight et al. further disclose coating processes ranging fromdipping/painting, chemical adsorption and thermal spraying. Bryan et al.(U.S. Pat. No. 5,301,211) disclose a linear magnatron sputteringapparatus to uniformly coat zirconium alloy nuclear cladding in anatmosphere of argon gas. A variety of coating materials are mentioned,including TiN, TiAlN, TiC and TiCN. Coker et al. (U.S. Pat. No.4,049,841) generally teach plasma and flame spraying techniques.

Cabrero et al. (U.S. Patent Application Publication No.US2011/0170653A1) disclose cladding totally or partially made of acomposite of a SiC ceramic fiber matrix, in the form generally of randomorientation, weaves, knits or felts. This can include severalsuperimposed layers. This matrix includes a carbide, for example, TiCand Ti₃SiC₂.

What is needed is a new type of protective means; a main object of thisinvention is to provide this and solve the problems described above.

SUMMARY

The above problems have been solved and objects accomplished byproviding a zirconium alloy nuclear reactor cylindrical cladding,subject to a nuclear reactor environment, the cladding having an innersurface and inner volume of zirconium alloy, an outer surface and outervolume of a protective material, selected from the group consisting ofTi—Al—C ceramic, iron-based alloy, a Nanosteel Super Hard Steel® classof materials (herein referred to as Nanosteel®), or an alloy comprisingonly a mixture of Zr—Al, and an integrated middle volume of zirconiumoxide, zirconium, and in excess of the sound velocity impactedprotective material where the highest density of protective material isat the cladding outer surface to protect the cladding from reactorenvironment and any further oxidation of the zirconium, where theintegrated middle volume provides structural integrity for the cladding.

The invention also resides in a method of forming an integrated gradientnetwork of protective particles into the ZrO₂ layer and base Zr tube ofnuclear reactor cladding comprising the steps of: providing a Zr alloynuclear reactor cladding having a ZrO₂ layer; providing a protectivematerial; optionally heating the nuclear reactor cladding; loading theprotective material into a hybrid thermal-kinetic deposition or coldthermal spray apparatus; and impacting at high velocity the nuclearreactor cladding with the protective material to impact through the ZrO₂layer and into the base Zr alloy to provide an integrated gradientnetwork of protective particles, protective particles plus the ZrO₂ andZr and a base Zr.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is an idealized schematic cross-section of a prior art protectivecoated nuclear cladding;

FIG. 2 is an idealized schematic cross-section of the protectiveintegrated gradient network mixing with ZrO₂ and penetrating into the Zrsubstrate;

FIG. 3A is one embodiment of a schematic cross-section of a zirconiumalloy nuclear sheath composite having at least two sheaths, thecomposite containing zirconium oxide scale on its outside surface; fuelpellets would be contained in the center of the sheath;

FIG. 3B is a schematic cross-section of the sheath of FIG. 3A where, forsake of simplicity, one sheath is shown; the sheath being impinged bytitanium-based or iron-based particles, or Zr—Al alloy particles at ahigh velocity from a particle source onto a heated zirconium surface ofFIG. 3A;

FIG. 3C is the finalized zirconium alloy nuclear sheath of FIG. 3B wherethe particles impinge the oxide coated tube and ultimately crater intothe zirconium by penetrating and mixing with the oxide scale, forming agradient of particles into the intermediate/middle layer of thezirconium alloy sheath; and

FIG. 4 illustrates a schematic flow diagram of the method of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

We have discovered a hybrid kinetic-thermal deposition process inconjunction with Ti—Al—C ceramics (such as Ti₂AlC or some otherelemental variant thereof) or iron-based alloys (which may be amorphous,semi-amorphous, or metallic alloys that may contain additional elementssuch as Al or C or Cr), Nanosteel®, or a Zr—Al alloy that can be used toform an integrated gradient layer comprised of the ceramic or metallicalloy mixed with the surface oxide, which penetrates in the zirconiumsubstrate to form a robust adherent matrix that protects the zirconiummetal from destructive bulk oxidation when exposed to reactorconditions. This deposition approach uses a combination of heat andkinetic energy to propel the ceramic or metallic alloy into the surfaceof the substrate. The material may be heated above its melting pointduring the deposition process, however, this is not a functionalrequirement of forming the gradient layer, and as such, the embodimentof this invention includes deposition below or above the melting pointof the deposition material. A schematic of the deposition technique isshown in FIGS. 2 and 3.

In general, the invention utilizes thermal-kinetic deposition (includinga cold spray application) to form mixed iron-based glassyamorphous/semi-amorphous/metallic alloy-ZrO₂ gradients or mixed Ti—Al—Cceramic-ZrO₂ or Nanosteel® gradients, or Zr—Al alloy gradients into, notjust onto, the surface of nuclear grade zirconium cladding. Oxidationresistant iron based alloys or Ti—Al—C based ceramics, or alloys ofZr—Al are distributed directly into/within/penetrating the oxide layerthat is present on all unprotected zirconium surfaces. The presence ofthis deposited network results in a gradient emanating from the claddingsurface which effectively eliminates bulk oxidation and hydriding of thezirconium upon exposure to pressurized water (PWR) or boiling waterreactor (BWR) conditions. The deposition technique itself is a hybridthermal-kinetic or cold spray process in which the materials are heatedand propelled in some optimized fashion towards the Zr substrate. Theterm “hybrid kinetic-thermal deposition” is defined as a process inwhich a high velocity gas propels particles of the protective materialinto the surface oxide and underlying bulk zirconium layers at avelocity greater than sound (>340 m/s). The particle sizes are chosen tobe large enough to deeply penetrate the boundary layer formed by theflowing gas jet around the tube, the oxide layer and the unoxidized tubealloy material, but small enough to interact with the structuralmaterial of the tube and the other protective particles to form animpermeable protective layer.

This may or may not melt the material as it is deposited, depending onthe application temperature. The thermal-kinetic or cold sprayapplication in combination with either of the aforementioned materialsresults in an oxide free interface between the zirconium and the reactorenvironment. As such, the zirconium cladding is imparted with enhancedcorrosion resistance, providing significant improvement in performanceand safety.

Applicable protective particle size is 1-500 micrometers for both coldor hot techniques. Cold spray temperatures are 250° C.-1,200° C.Material is propelled using a pressurized inert gas (to preventexcessive oxidation of the zirconium surface) such as N₂, He, or Ar.Typical spray velocities exceed the speed of sound >340 m/s. HVOF (highvelocity oxygen fuel) and application temperature is 800° C.-2,800° C.The velocity of the spray exceeds the speed of sound, >340 m/s,preferably 400 m/s to 1,200 m/s, most preferably 450 m/s to 1,000 m/s.Kerosene is one propellant material, while other species such aspropylene, acetylene, natural gas, or other combustible gases or liquidscan also be used. HVOF (or cold spray) may also be done whilesurrounding the substrate in an inert environment to reduce or eliminatesurface oxidation during deposition.

Referring now to FIG. 1 which shows prior art discrete coating on a ZrO₂layered ZrO surface. The surface in this case is the top half of anuclear cladding tube. The Zr substrate is shown as 10, and the ZrO₂layer (surface oxide) as 14 and the discrete top protective coatinglayer as 16.

FIG. 2 is a general schematic of the subject invention, with Zrsubstrate 10, deposited material mix of protective material+ZrO₂penetrating into the Zr substrate—all shown as 20 and high density outerportion of protective material as 22.

The coating approach of prior art FIG. 1 only applies material on top ofthe ZrO₂ layer, resulting in poor adhesion and potential failure. Theintegrated deposition technique of this invention, FIGS. 2 and 3, mixesoxidation resistant material directly into the ZrO₂ layer whichultimately penetrates into the Zr substrate itself resulting in strongadhesion and a dense oxidation resistant surface that protects theunderlying substrate from the reactor environment. There is substantial“cratering” of the material through the ZrO₂ clue to the kinetic forcewith which the protective material is applied. It arrives withsufficient kinetic energy/force to penetrate the thin oxide layer thatexists on the cladding surface.

FIGS. 3A-3C show the process and result of this invention in moredetail. FIG. 3A is a schematic of a cylindrical zirconium alloy nuclearreactor cladding 30 having an axis 32 in a normal environment of H₂Oduring operation or, normal ambient air, where invariably an oxidecoating 34 grows, in this case ZrO₂ since the cladding 30 is made ofgreater than 95% Zr alloy 36. The cylinder bore is shown as 38. This isthe starting situation. The oxide coating/layer 34 is generally 10nanometers thick, which translates to several atomic layers of ZrO₂ ifonly exposed to air and is comprised of a reasonably tight but stillporous ZrO₂ coating adhering to the top surface 40 of the claddingbefore use.

FIG. 3B illustrates the process of “high velocity,” meaning up to 3½times the speed of sound, impacting of the protective material 42 usingthermal-kinetic deposition techniques, which in some cases can be firstused to initially heat the reactor cladding to an outer top surfacetemperature from 200° C. to 400° C. in a non-oxidizing atmosphere, withthe interior remaining substantially cooler, and impact the protectivematerial at a velocity greater than the speed of sound >340meters/second, which velocity and heating combine to effect extremelyhigh impact directly through the ZrO₂ scale 34 and deep into the Zralloy 36 forming substantial cratering and an adherent network, shown inFIG. 3C.

The particle size of the protective material should be relatively largeto create massive impact and substantial cratering of the Zr alloy 36but small enough to allow particle to particle interactions to form atight, impermeable layer. The particle size is generally from 1micrometers to 500 micrometers, preferably from 10 micrometers to 100micrometers. Under 1 micrometers, the impact effect will be lesseffective leading to excessive particle loss and insufficientpenetration.

The protective particles can be either Ti_(x)Al_(y)C_(z) ceramic wherex=2 to 4; y =1 to 1 and z=1 to 3; or iron-based alloyFe_(x)Al_(y)Cr_(z)(G) where x=0 to 70, y=0 to 30, and z=0 to 30 and (G)is comprised of any number of minor constituents that may include theelements Ni, Si, Mn, Mo, P, S, Co, W, B, or C. The protective particlescan also be Nanosteel® which has the composition: material chemistry (wt%); Cr<25%; W<15%; Nb<12%; Mo<6%; B<5%; C<4%; Mn<3%; Si<2%; and Febalance. Additionally, the deposited particles could have a formulationthat is comprised of a Zr—Al alloy, where Al may comprise up to 99.9atom % of the alloy. However, the preferred protective particles areTi—Al—C, and the most preferred formulation is Ti₂AlC. Ti₂AlC ispreferred because it is corrosion resistant to >1,250° C. Theseintegrated protective layers also serve to improve the fuel reliabilityand the fuel cycle economics because they are hard and resist wear. Inaddition, these layers have a very high temperature capability thatenables better corrosion resistance, and consequently are moreaccident-tolerant at high temperature accident conditions.

Referring now to FIG. 3C, the final hybrid thermal-kinetic depositedprotective gradient nuclear reactor cladding. As can be seen, theprotective material 42 permeates/infiltrates as an integrated gradientnetwork. As can be seen, the protective material 42 has penetrated theoxide coating 34 shown in disarray and intermixed with the protectivematerial in an intermediate portion Z, where X is the total diameter ofthe nuclear reactor cladding 30, while Y is the diameter of thecylindrical bore 38 and Q is the outer portion mixture of ZrO₂ andprotective material with the highest density of protective materialbeing at the final exterior 44 of the nuclear reactor cladding. Reactorcoolant water is shown by arrow 46. As a means of judging distances, ifX−Y=1,000 units; 2 Z=1 to 10 units=impregnation; and Q=100 to 600 unitsof outer particles+oxide+Zr alloy.

Referring to FIG. 4, the method of the invention is schematically shown.In that FIG. 4, a cladding material is provided 60. Optionally, heating62 the nuclear reactor cladding. Loading protective material into ahybrid thermal-kinetic deposition or cold thermal spray apparatus 64.Impacting at high velocity 66 the nuclear reactor cladding with theprotective material, to impact through the ZrO₂ layer and into the baseZr tube as shown in FIG. 3C; providing an integrated gradient network 68of protective particles, protective particles plus ZrO₂ and Zr and abase Zr.

EXAMPLES

Multiple zirconium “coupons” that were 48 mm long, 10 mm wide, and 3 mmthick were deposited with oxidation resistant material using the processdescribed in the previous paragraphs. Ti₂AlC was deposited using HVOF(high velocity oxygen fuel) on the coupons at a flame temperature ofabout 5,000° F. (2,760° C.), although the particle temperature did nottend to reach this value. Kerosene was used as the fuel. The Ti₂AlCparticle size range was 10 μm-60 μm and the spray velocity wasapproximately 2,000 ft/s-2,700 ft/s (600 m/s-800 m/s). The nozzletechnology used in this spray process mimics that of a rocket engine.

Nanosteel® powder with a nominal size of 15 μm-53 μm was applied using acold spray (which is also a type of hybrid thermal-kinetic depositionmethod) technique with a deposition temperature that ranged between 932°F.-1,652° F. (500° C. and 900° C.), although the particle temperaturedid not tend to reach this value. The cold spray particle velocity had arange between 2,230 ft/s and 3,500 ft/s (680 m/s-1,050 m/s) and wasexecuted using pressurized nitrogen gas.

In both applications, the zirconium coupons were not purposely heatedduring the deposition process. The zirconium coupons were then placed inan autoclave for 28 days at 800° F. (426.6° C.) and 1,500 psia tosimulate accelerated exposure to the high-temperature and pressureconditions of a nuclear reactor. The results of these autoclave testsshow that the deposition technique in concert with materials describedabove can prevent bulk oxidation of the zirconium surface in a reactorenvironment. Photomicrographs showed a gradient/impregnation impinginginto the zirconium “coupons” mixing with the zirconium oxide, as shownin FIGS. 2 and 3C.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

What is claimed is:
 1. A method of forming an integrated gradient network of protective particles into a ZrO₂ layer and a base Zr tube of a nuclear reactor cladding, the integrated gradient network having an inner surface and an inner volume of a layer of zirconium alloy, an outer surface and an outer volume of a layer of a protective material and an integrated middle volume of a layer of a combination of zirconium oxide, zirconium and excess sound velocity-impacted protective material, the method comprising the steps of: a) providing a Zr alloy nuclear reactor cladding having a Zr base alloy layer and a ZrO₂ outer layer; b) providing a protective material comprised of Zr—Al alloy particles; c) loading the protective material into a hybrid thermal-kinetic spray deposition or cold spray apparatus; and d) impacting the nuclear reactor cladding with the protective material to impact at a velocity greater than sound and sufficient to penetrate through the ZrO₂ layer and into the Zr base alloy layer to provide an integrated gradient network of a layer of the protective particles, the layer of the combination of protective particles, ZrO₂ and Zr and the Zr base alloy layer; wherein the highest density of protective material is at the cladding outer surface to protect the cladding from the reactor environment and any further oxidation of the zirconium; and wherein the integrated middle volume provides structural integrity for the cladding.
 2. The method of claim 1, wherein the impacting velocity is 3½ times greater than 340 m/s.
 3. The method of claim 1 further comprising heating the nuclear reactor cladding prior to impacting the nuclear reactor cladding with the protective material.
 4. The method of claim 3 wherein the heating step heats an outer surface of the nuclear reactor cladding between 200° C. to 400° C.
 5. The method of claim 1 wherein the impacting step is carried out with a hybrid thermal-kinetic deposition process.
 6. The method of claim 1 wherein the impacting step is carried out with a cold spray deposition process.
 7. The method of claim 6 wherein the cold spray process is carried out at a temperature between 250° C. and 1200° C.
 8. The method of claim 1 wherein the protective material has a particle size of approximately between 1 and 500 micrometers.
 9. The method of claim 1 wherein the impacting step is performed in an inert environment. 